1. Field of the Invention
This invention relates to radio frequency (RF) transceivers, and particularly to portable, fast-hopping RF transceivers.
2. Description of the Related Art
Many modern communications systems employ the concept of xe2x80x9cfrequency hoppingxe2x80x9d, in which the frequency of a transmitted signal is changed at a rapid rate. In some frequency-hopping systems, the rate at which a signal frequency f changes with time t, i.e., xcex94f/xcex94t, is defined as a xe2x80x9cchirp ratexe2x80x9d. Data is conveyed in a series of pulses, which can be properly demodulated (or xe2x80x9cdechirpedxe2x80x9d) only when the chirp rate is known.
The chirp rate of an incoming RF signal is conventionally determined (referred to herein as xe2x80x9cchirp acquisitionxe2x80x9d) using the receiver portion of a transceiver: the incoming signal is mixed with a local oscillator (LO) signal having a frequency which is varied. The resulting intermediate frequency (IF) is processed to detect the LO frequency needed to dechirp the input signal. The incoming RF signals are high frequency, as are the corresponding LO signals needed to detect the chirp rate and to dechirp the input signal. These high frequency LO signals are typically generated with a direct digital synthesizer (DDS) driven with a phase-locked-loop (PLL); as such, there is a significant amount of power associated with the generation of the LO signals. This high power consumption may unacceptably shorten the operational life of battery-powered field transceivers.
A low power chirp acquisition mode and chirp acquisition method for a fast-hopping RF transceiver are presented which overcome the problems noted above.
The invention provides a low power, fast-hopping LO generator for the chirp acquisition process. Initially, only pulse detection circuitry is powered. When an incoming pulse is detected, a low power chirp acquisition mode is initiated. While in chirp acquisition mode, all transceiver circuitry not required to determine the chirp rate is powered down. A low power fast-hopping LO generator is powered up to provide one or more LO signals to demodulate the incoming signal, and an active bandpass filter connected to filter the demodulated output is arranged to extend the width of its passband to include the chirp rate. The filtered signal is digitized with an analog-to-digital converter (ADC) and processed to determine the incoming signal""s chirp rate.
To determine chirp rate, the low power LO generator must generate LO signals having different frequencies. This is preferably accomplished with the use of a look-up table, which produces a plurality of digital output word sequences in a predetermined order in response to a clock signal having a low frequency relative to the chirp rate. Each digital output word sequence represents a respective discrete LO frequency. The digital word sequences are provided to a sine-weighted digital-to-analog converter (DAC) which produces an analog output signal in response, with the frequency of the analog output signal changing with each digital word sequence. The varying frequency analog output signal is multiplied to produce the discrete LO signals provided to the mixers. The ADC and signal processor are also powered during chirp acquisition mode, with the signal processor determining the chirp rate when an LO signal of appropriate frequency is applied to the mixer. Once the chirp rate is detected, the low power LO generator is powered down, the passband of the active bandpass filter is narrowed, and the remaining receiver circuitry is powered up to dechirp the RF input signal.